The subject matter disclosed herein relates to X-ray tubes, and in particular, to X-ray cathode systems and X-ray cathodes.
Presently available medical X-ray tubes typically include a cathode assembly having an emitter and a cup. The cathode assembly is oriented to face an X-ray tube anode, or target, which is typically a planar metal or composite structure. The space within the X-ray tube between the cathode and anode is evacuated.
X-ray tubes typically include an electron source, such as a cathode, that releases electrons at high acceleration. Some of the released electrons may impact a target anode. The collision of the electrons with the target anode produces X-rays, which may be used in a variety of medical devices such as computed tomography (CT) imaging systems, X-ray scanners, and so forth. In thermionic cathode systems, a filament is included that may be induced to release electrons through the thermionic effect, i.e. in response to being heated. However, the distance between the cathode and the anode must be kept short so as to allow for proper electron bombardment. Further, thermionic X-ray cathodes typically emit electrons throughout the entirety of the surface of the filament. Accordingly, it is very difficult to focus all electrons into a small focal spot.
X-ray systems typically include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The data acquisition system then reads the signals received in the detector, and the system then translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
X-ray tubes typically include a rotating anode structure for the purpose of distributing the heat generated at a focal spot. An x-ray tube cathode provides an electron beam from an emitter that is accelerated using a high voltage applied across a cathode-to-anode vacuum gap to produce x-rays upon impact with the anode. The area where the electron beam impacts the anode is often referred to as the focal spot. Typically, the cathode includes one or more cylindrically wound filaments positioned within a cup for emitting electrons as a beam to create a high-power large focal spot or a high-resolution small focal spot, as examples. Imaging applications may be designed that include selecting either a small or a large focal spot having a particular shape, depending on the application.
Conventional cylindrically wound filaments, however, emit electrons in a complex pattern that is highly dependent on the circumferential position from which they emit toward the anode. Due to the complex electron emission pattern from a cylindrical filament, focal spots resulting therefrom can have non-uniform profiles that are highly sensitive to the placement of the filament within the cup. As such, cylindrically wound filament-based cathodes are manufactured having their filament positioned with very tight tolerances in order to meet the exacting focal spot requirements in an x-ray tube. In particular, X-ray tubes including coiled or cylindrically wound filament-based cathode designs require that the cathode be seated deeper within the cup in order to provide sufficient distance between the cathode and the anode for proper focusing of the more scattered electron emission from the coiled electrode.
In an attempt to generate a more uniform profile of electrons emitted from the cathode towards the anode to obtain a more uniform focal spot, wound filament-based cathodes having an approximately flat emitter surface have been developed. Typically a flat emitter may take the form of a D-shaped filament that is a wound filament having the flat of the “D” facing toward the anode, such as disclosed in U.S. Pat. No. 7,795,792 B2, incorporated herein by reference in its entirety. Such a design emits a more uniform pattern of electrons and emits far fewer electrons from the rounded surface of the filament that is facing away from the anode (that is, facing toward the cup). D-shaped filaments, however, are expensive to produce (they are typically formed about a D-shaped mandrel) and typically require very tight manufacturing and positioning tolerances with separately biased focus electrodes in order to meet focal spot requirements.
As an alternative to the coiled emitters, a flat surface emitter (or a ‘flat emitter’) may be positioned within the cathode cup with the flat surface positioned orthogonal to the anode, such as that disclosed in U.S. Pat. No. 8,831,178, incorporated herein by reference in its entirety. In the '178 patent a flat emitter with a rectangular emission area is formed with a very thin material having electrodes attached thereto, which can be significantly less costly to manufacture compared to conventionally wound (cylindrical or non-cylindrical) filaments and may have a relaxed placement tolerance when compared to a conventionally wound filament. In particular, cathodes with flat emitters are capable of directing the electrons in a more parallel direction from the emitter, and thus can be seated more shallowly within the cup.
X-ray tubes having cathodes with flat emitters may additionally include a grid electrode. The electron emission originating from the surface of a thermoionic electron emitter, the flat emitter, strongly depends on the “pulling” electric field generated by the X-ray tube's anode. For enabling fast on/off switching of the tube, it is known from the relevant prior art that X-ray tubes of the rotary-anode type may be equipped with a grid electrode placed in front of the electron emitter. To shut off the electron beam completely, a bias voltage is applied to the grid electrode which generates a repelling field and is usually given by the absolute value of the potential difference between the electron emitter and the grid electrode. The resulting electric field at the emitter surface is the sum of the grid and the anode generated field. If the total field is repelling on all locations on the electron emitter, electron emission is completely cut off.
However, X-ray tubes using a flat filament/emitter require higher negative voltages to be applied to the grid electrode compared to cathodes that utilize coiled filaments due to the geometry or shape of the flat emitter. Thus, if an X-ray tube includes a cathode with a flat emitter and both grid and focusing electrodes, large bias voltages for the grid electrode are required in excess of the typical +/−10 kV with respect to the cathode potential in order to accurately focus the electron beam from the emitter on the desired target size. Such large voltages present many difficulties for the construction of X-ray tubes, including reliability and construction cost challenges regarding the requirements for the insulation located both inside the cathode assembly and on the supply cable.
Nevertheless, flat emitters are desirable for use in X-ray tube cathodes primarily for the longer filament life provided by the flat emitter geometry. However, in order to limit the required grid voltage to within typical ranges, the aspect ratio of the filament has to be increased. In so doing, the bias voltages for the focusing electrode(s) need to become positive to reduce the size of the electrode beam from the emitter to obtain useful focal spot sizes. However, cathodes positive voltages draw electrons and make it impossible to focus the electron beam and/or obtain a stable bias voltage.
Hence it is desirable to provide an X-ray tube with a flat filament emitter/cathode which can effectively employ focus and grid electrodes within a typical bias voltage maximum of +/−10 kV with respect to cathode while also providing a large emission area.